This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. P2001-213280, filed on Jul. 13, 2001; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to high frequency communications equipment. Particularly, the present invention relates to a high frequency filter for passing only a desired signal frequency band.
2. Discussion of the Background
Communications equipment for sending and receiving information wirelessly or by wire is made up of various high frequency devices such as amplifiers, mixers, and filters. Many high frequency devices utilize their resonance characteristics. For example, a bandpass filter includes an array of resonators and has a function of passing signals only in a certain frequency band.
Bandpass filters in communications systems are required to have skirt characteristics such that interference is eliminated between adjacent frequency bands. Skirt characteristics relate to the degree of attenuation over a range of frequencies from an end of a pass frequency band to a stop frequency band. In particular, when a bandpass filter having steep skirt characteristics is used, frequency signals outside the pass frequency band can be strictly eliminated. Accordingly, the frequency band can be divided into plural sections and effectively utilized.
A first requirement for realizing a filter having steep skirt characteristics is that a resonator forming the filter accomplishes a high unloaded Q value. For this purpose, a substrate forming the filter needs to have a small dielectric loss.
Furthermore, if a superconductor is used as the conductor forming the resonator, the conductor loss is quite small. As a result, a quite high unloaded Q value can be accomplished.
Conventionally, LaAlO3 and MgO have been chiefly used as substrates employed for filters. These substrates have dielectric constants of about 10xe2x88x926, which are relatively small values.
However, an LaAlO3 substrate is disadvantageous because the dielectric constant across the substrate is not uniform due to the crystal having a twin boundary. Further, MgO is disadvantageous because of its deliquescence and vulnerability to moisture and water.
Alternatively, sapphire substrates may be used as substrates employed for filters. A sapphire (Al2O3) substrate has a relatively small dielectric loss of 10xe2x88x927 to 10xe2x88x928. Also, the crystal structure of a sapphire substrate is stable, and the dielectric constant across the substrate is stable. A sapphire substrate also has a stronger mechanical strength than a MgO substrate and is easier to handle. Additionally, it has the advantage of being much cheaper than LaAlO3 and MgO substrates. Sapphire substrates are also higher in thermal conductivity than LaAlO3 and MgO substrates. Accordingly, when a superconductor is used as a conductor and cooling is necessary, the temperature distribution is small and sapphire substrates are advantageous for more stable operation. Accordingly, the sapphire substrate has good characteristics as a substrate for a filter. Sapphire substrates include substrates obtained by cutting out (1-100)-plane (M-plane 11) shown in FIG. 1A and substrates obtained by cutting out (1-102)-plane (R-plane 12) shown in FIG. 1B.
However, a sapphire crystal has a hexagonal system and its dielectric constant is anisotropic. Accordingly, the designing of a circuit utilizing a sapphire substrate is problematic due to the difficulty to design a circuit. Further, a sapphire substrate is problematic when a superconductor is used as a semiconductor, as it is difficult to form good-quality, high-temperature semiconductive film on the M-plane 11.
R-plane substrates have the advantage of being cheaper than M-plane substrates and that good-quality high-temperature superconductive films can be formed on R-plane substrates. However, R-plane substrates are problematic as they increase the size of a device. Further, R-plane substrates are relatively costly, especially when a superconductor is used as a conductor. The increase in size of the device is attributed to forward-coupled filters, filters using meander open-loop resonators, and quasi-lumped element filters that must have many resonators to realize steep skirt characteristics.
Accordingly, there is a demand for a hairpin type filter formed on a sapphire R-plane or an improved filter that is based on a hairpin type filter. Generally, non-diagonal elements of dielectric constant tensor always contribute on a sapphire R-plane. Therefore, the effects of impedance mismatching differ greatly depending on the geometry of the resonator and on the direction of installation of the resonator. Accordingly, where a sapphire R-plane is used, appropriate geometry and installation direction of the resonator are not previously known. Hence, a small-sized filter has not been previously accomplished.
Embodiments of the present invention provide a high frequency filter. The filter comprises a substrate, a conductive layer, a pair of input terminals, an output terminal, and resonating portions. The substrate has a first face and a second face. The first face is a sapphire R-plane. The conductive layer is on the second face of the substrate and is connected to a ground level. The pair of input terminals and the output terminal are formed on the first face of the substrate. The resonating portions are formed between the pair of the input terminals and the output terminal. The resonating portion has a hairpin-shape and a longer side. The longer side makes an angle of "psgr" with  less than 11-20 greater than  direction of the first face. Angle "psgr" is xe2x89xa70xc2x0 and xe2x89xa630xc2x0.
Embodiments of the present invention relate to a high frequency filter comprising a substrate, conductive layer, a pair of input terminals, output terminal, and resonating portions. Resonating portions are formed between the pair of input terminals and the output terminal and have an asymmetric shape.
Embodiments of the present invention alleviate the disadvantages, which are discussed above, of the background art. Accordingly, the embodiments of the present invention comprise a substrate having a relatively uniform dielectric constant across a substrate. The present invention is relatively resilient to moisture and water. Further, the size of the device comprising the embodiments of the present invention is relatively small and can be manufactured in a cost effective manner.